Effect of Unidirectional Vertical Wind Shear on Tropical Cyclone Intensity Change—Lower-Layer Shear Versus Upper-Layer Shear

Abstract

In this study, a quadruply nested, nonhydrostatic tropical cyclone (TC) model is used to investigate how the structure and intensity of a mature TC respond differently to imposed lower-layer and upper-layer unidirectional environmental vertical wind shears (VWSs). Results show that TC intensity in both cases decrease shortly after the VWS is imposed but with quite different subsequent evolutions. The TC weakens much more rapidly for a relatively long period in the upper-layer shear than in the lower-layer shear, which is found to be related to the stronger storm-relative asymmetric flow in the middle-upper troposphere and the larger vertical vortex tilt in the former than in the latter. The stronger storm-relative flow in the former imposes a greater ventilation of the warm core in the middle-upper troposphere, leading to a more significant weakening of the storm. The storm in the lower-layer shear only weakens initially after the VWS is imposed but then experiences a quasi periodic intensity oscillation with a period of about 24 hr. This quasi periodic behavior is found to be closely related to the boundary layer thermodynamic “discharge/recharge” mechanism associated with the activity of shear-induced outer spiral rainbands. There is no significant intensity oscillation for the storm embedded in the upper-layer shear, even though outer spiral rainbands develop quasi periodically also. The boundary layer inflow is very weak in that case and the low equivalent potential temperature air induced by downdrafts in outer spiral rainbands therefore cannot penetrate into the inner core but remains in the outer region.